(EXAM WITH COMPLETE SOLUTIONS) # 3 Review Questions
A blackbody glowing with which of the following colors is hottest?
Of the following photons, which has the lowest energy?
The spectrum of which of the following objecting will show a blueshift?
What is a blackbody? ? If stars appear to be like...
(EXAM WITH COMPLETE SOLUTIONS) # 3
Review Questions
1). A blackbody glowing with which of the following colors is hottest?
Ans: Violet
2). Of the following photons, which has the lowest energy?
Ans: Infrared
3). The spectrum of which of the following objecting will show a blueshift?
Ans: An object moving directly toward Earth
4). What is a blackbody? ? if stars appear to be like blackbodies, why are they not black?
Ans: A blackbody is an object that absorbs all of the enrgy that falls on it. According to
the laws of physics it therefore re-radiates thermal radiation with the blackbody spectrum
with 100% efficiency. The nature of a blackbody is that it radiates electromag- netic
radiation. This radiation is in the visible range for a temperature of several thousand
kelvin.
5). What does it mean to say that a star appears almost like a blackbody
Ans: Stellar surfaces emit light that is close to an ideal black body...Stars radiate with
an efficiency around 85%, so behave as blackbodies.
6). What is wien's law? how could you use it to determine the temperature of a star's surface?
Ans: A Law that states that the temperature of a blackbody is inversely proportional to
the peak wavelength of the blackbody spectrum.
An observer can take a spectrum of a star and find the peak wavelength in meters
Because a star has a spectrum that is similar to a blackbody, Wien's Law can be used.
Plugging this wavelength into the right side of this equation (T= 2.93 x 10^-3 m.K / `A
max), one can solve for the temperature
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, 7). What is the stefan-boltzman law? how do astronomers use it?
Ans: The Stephan-Boltzmann Law states that the total radiated energy of an object (or
every square meter thereof) increases proportional to the fourth power of the absolute
(Kelvin) temperature. Astronomers use this to determine immediately the power per
square meter the star emits. This gives a relation between the size of the star and its
luminosity. We will see in chapter 11 that this is used to determine the size of the star.
8). Using wien's law and the stefan-boltzmann law, state the changes in color and intensity that
are observed as the temperature of a hot, glowing object increases
Ans: As the temperature of a hot, glowing object ( a blackbody) increases, two things
occur. First, the wavelength of the most intense light (the peak wavelength) shifts to
shorter wavelengths. This causes the color to shift from the red side of the visible color
rainbow towards the violet side. Seconf the energy flux(the intensity) of light emitted over
all wavelengths increases by the fourth power of the increase in temperature. For
example, an increase in the temperature by a factor of two would mean an increase in the
total energy flux by a factor of 16.
9). What color will an interstellar gas cloud composed of hydrogen glow, and why?
Ans: A cloud of gas will glow pink or red (personal perception) if it is illuminated by a
hot star that emits a lot of ultraviolet. The color is dominated by the n = 3 to n = 2
transition in hydrogen, which emits red light (Hα, λ = 656 nm), but with a significant
amount of n = 4 to n = 2, which emits blue (Hβ , λ = 486 nm). (Dozens of other transitions
are also present.) The ultraviolet illumination is necessary to excite the hydrogen above
the n = 1 level in the first place. See the Fig 12-16 on page 387 (9th edition). A cloud
containing dust (a reflection nebula) will glow blue if illuminated by white light. This is
because the dust scatters blue light more than red, so the scattered light that reaches
Earth is dominantly blue. The same thing happens in Earth's atmosphere to make the sky
blue.
10). What is an element? list the names of five different elements, and briefly explain what
makes them different from each other.
Ans: A class of atoms which all contain the same number of protons in their nuclei.
Hydrogen-the lights element with only one proton in the nucleus.
Carbon- the number of protons(6) determines that carbon can react easily with many
elements and also form long chains of carbon. These carbon chains are key component
to life.
Xenon- a very heavy gas with 54 protons. The protons number determine that the gas
forms weak and infrequent chemical bonds.
Plutonium- a heavy radioactive metal with 94 protons. The protons clumped tightly
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, together within the large nucleus repel one another so much that the atoms are unstable
and break apart naturally, releasing energy through nuclear fission.
Mercury- a liquid that exhibits metallic properties due to its 80 protons.
11). How are the three isotopes of hydrogen different from each other?
Ans: Isotopes of an element are different because the number of neutrons in the
nucleus is different. The three isotopes of hydrogen (hydrogen, deuterium and tritium) are
different in mass. They are also different in terms of how frequently they appear in nature;
hydrogen is the most abundant elements isotope in the universe, while deuterium is much
rarer and tritium is extremely rare.
12). Explain how the spectrum of hydrogen is related to the structure of the hydrogen atom.
Ans: The structure of the hydrogen atom is
a. that the electron can orbit only in certain orbits (n = 1, 2, 3, 4, ...);
b. that each orbit has a characteristic energy;
c. that the hydrogen spectrum consists of photons whose energies are the differ-
ences in energy between allowed transitions; and
d that the energy of a photon automatically determines its wavelength (a uni-
versal characteristic of light, not specific to hydrogen).
13). Why do different elements have different patterns of lines in their spectra?
Ans: Electrons in an atoms are attracted tot he positively charged, proton-filled nucleus
through the electromagnetic force. There are fixed amounts of energy that a bound
electron can have, and as the electron moves between these energy levels, they emit
photons. Each atomic element has a different spacing between energy levels, depending
on the number of protons, neutrons and electrons in the atom. Therefore, each atomic
element emits photons in a unique emission spectrum.
14). Explain why the doppler shift tells us inly about the motion directly along the line of sight
between a light source and an observer, but not about motion across the celestial sphere.
Ans: The Doppler shift tells us about motion along the line of sight because motion
along the line of sight stretches or compresses the waves. Motion across the line
of sight does not stretch or compress the waves, so it cannot be detected, except by
waiting for the object to move far enough to measure.
15). What is the doppler shift, and why is it important to astronomers?
Ans: The doppler shift is the stretching or compressing of waves when the wave
source is moving away or towards the observer. The effect occurs in all wave phenomena,
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